MEMS sensors, such as MEMS capacitive microphones, are becoming increasing popular, at least partly due to their small size. For instance MEMS microphones may usefully be employed on portable electronic devices such as mobile telephones or tablet computers and the like. The increasing popularity of voice control is also leading to microphones being provided on a range of devices, such as smart watches or other wearable devices or on other consumer electronics products and MEMS microphones are being usefully used on such products.
MEMS capacitive microphones typically comprise one electrode, which is moveable with respect to at least one fixed electrode in response to incident acoustic waves to form a variable capacitance, typically of the order of 1 pf or so. The moveable electrode may, for example, be supported by a flexible membrane. In use a first one of the electrodes may be biased by a relatively high stable bias voltage VBIAS, say 12V or so in some instances, whilst the second electrode is biased to another fixed voltage VREF, typically ground, via a very high impedance, for example, in the order of 10 GΩ. Acoustic waves incident on the capacitive transducer will cause displacement of the moveable electrode with respect to the fixed electrode, thus changing the spacing between these electrodes and hence the inter-electrode capacitance. As the second electrode of the transducer is biased via a very high impedance, these changes in capacitance cause a signal voltage to appear at the input terminal. Given the small capacitance of the MEMS sensor the input signal is relatively small and thus the signal is amplified by a low-noise amplifier arrangement.
One issue that arises for such MEMS microphones is providing sufficient dynamic range. To provide acceptable output signal levels at lower input acoustic signal level requires a certain amplifier gain. However at higher acoustic signal levels this can result in overload, where the resulting relatively large input signal magnitude exceeds the linear range of the amplifier at the input and/or output and distortion is introduced. In most cases the maximum signal level that can be amplified is limited by the power supply voltage minus headroom.
One way to deal with this problem is to use compression, where the system parameters are adjusted to effectively reduce sensitivity of the sensor based on an indication of signal amplitude, for instance the bias voltage supplied to the first electrode may be reduced. However this adds complexity and results in a variable sensitivity over the operating range.